The operation of the blast furnace in the production of iron involves processes of chemical reduction in which oxides of iron and other metals are reduced and oxygen removed. The blast furnace is charged with four basic ingredients: (1) iron oxides, in the form of raw ore, beneficiated pellets, briquettes, nodules, sinters, or other agglomerates; (2) calcium carbonate (the term calcium carbonate is used to include either limestone or dolomite); (3) a fuel usually in the form of coke; and (4) air which provides oxygen to support the combustion. The raw iron as it comes from the Lake Superior region has contained approximately 50 percent iron in the form of iron oxide (Fe.sub.2 O.sub.3), with the remainder being silica (SiO.sub.2), aluminum (Al.sub.2 O.sub.3), magnesia (MgO), lime (CaO), sulfur (S) and phosphorous (P), and manganese oxide (MnO). The sulfur and phosphorous are commonly considered impurities.
The iron oxides, or other metallic charged materials, coke and calcium carbonate are charged into the blast furnace, one at a time, in measured amounts, to form layers of iron ore, limestone or dolomite, and coke; and air (wind) is passed through these layers and the coke is burned. Burning of the coke produces heat and carbon monoxide which has a part in the chemical reduction of the iron oxides. As the coke burns the iron oxides are reduced and come into the form of molten iron. The limestone or dolomite, along with quantities of impurities such as sulfur and phosphorous form a slag. The hearth which is located in the lower part of the furnace, is the hottest part of the furnace and the layers of ore, coke and calcium carbonate keep moving downwardly within the furnace to the hearth.
At some point in this movement downwardly in the furnace slag is formed, and after its full passage downwardly in the furnace it is withdrawn from the furnace in the form of liquid slag. The slag is important to the operation of the furnace because it carries with it many unwanted impurities and so separates these from the iron and removes them from the furnace.
When the downward movement of the iron bearing charged materials, the coke and the calcium carbonate proceeds in a uniform way with the movement taking place constantly and evenly on all sides of the furnace, this is evidence of good operation. Unfortunately, this is not always the case.
As is well known to blast furnace operators there are times when the downward movement of the ingredients charged into the furnace is not regular and uniform or when the movement at some place within the furnace is greater than at other places, making the furnace unbalanced. There are even times when at substantial areas the movement becomes restricted, and then after operation for a time under such conditions the whole mass may let loose, descending at once into the hot part of the furnace with the result that the hearth temperature is reduced below an operable temperature, sometimes almost extinguishing the fire. When this happens, the furnace may have to be shut down, cleaned and restarted, which is a time-consuming and expensive operation.
It is our belief that the faulty operation above referred to is due in large part to the presence in the charged materials of alkali metal oxides such as Na.sub.2 O, K.sub.2 O and Li.sub.2 O. These oxides appear to pass downwardly to hotter parts of the furnace and there become volatilized after which they pass upwardly in the furnace with the wind and then condense above the mantle of the furnace forming stable alkali-alumino silicates. Such silicates are believed to lead to a scaffolding effect which prevents the layered burning material from descending in a regular, uniform manner. A continuation of this action develops a situation where the mass will collapse of its own weight, chilling the furnace hearth where the most important smelting reactions take place.